This invention relates to the manufacture of flexible plastic film, such as polyethylene film, for use as bags, sheets, and the like.
Polyethylene film is typically created using a tube extension process, in which a tube of the so plastic is extruded though an annular die, and is then inflated and expanded until the tube wall is a desired thickness. The film is typically made anywhere in the range of about 0.3 mils to several mils, depending upon the end use. After the tube of material is extruded, it is drawn vertically upward, typically a distance of forty yet or more. The air within the tube expands the tube outward until the plastic sets. On its journey upwards, the extended tube has to be flattened so that it may pass between rollers. The flattening is accomplished by a collapsing assembly, in which two facing arrangements of collapsing boards urge the tubular extrusion to a flattened state by the time it reaches the rollers.
Typically, this arrangement consists of a pair of facing collapsing frames, each of which has a number of collapsing boards which are horizontal and arranged in parallel, one above the other on the frame. There are typically sixty to ninety boards on a side, the boards being from twelve inches to fourteen feet in length. These collapsing boards are, by convention, formed of high quality hardwood, usually a close grained maple, which is extremely expensive. The boards have a curved or arcuate face against which the plastic film passes. The polyethylene film is quite abrasive to the wood. Even very high quality hardwood as typically used will tend to wear out somewhat unevenly. These boards thus have to be replaced periodically and this is an expensive and time consuming procedure. To replace the collapsing boards on the frame, the boards and any associated hardware have to be unscrewed, so that the entire board can be replaced. The boards are typically four to twenty feet in length, and can be quite heavy and unwieldy for a workman to manage. There is considerable down time involved with collapsing board replacement, as it now takes about three full days to replace all the collapsing boards.
A solution to the problem just described has been disclosed in my earlier U.S. Pat. No. 4,943,226, which was granted on Jul. 24, 1990. That patent describes replaceable plastic covers or wear plates to be installed on the collapsing boards or slats. These covers are extruded of a rigid or semi-rigid plastic resin material impregnated with graphite or another lubricating agent. In this system, the collapsing board covers have a front face or web that is contoured to match the front thee of the collapsing boards, and upper and lower flanges that fit behind the board to retain the cover on it. The covers can snap fit onto the board or can slide on. The covers absorb the wear and abrasion from the polyethylene film, and when worn can be quickly and economically replaced.
However, even in this case the initial requirement for close-grained maple is a great capital expense. Also, for longer slats, it is often necessary to join maple lengths end to end. In addition, the heat from friction with the polyethylene film is difficult to dissipate in the maple slats. This can affect and limit the performance of the film extrusion process.